The production of chlorine by electrolysis of alkali chloride solutions is currently carried out by means of three different processes, namely the ion-exchange membrane process, the porous diaphragm process and the mercury cathode process. In the following, reference will be made, for the sake of simplicity of description, to the electrolysis of sodium chloride solutions. Nevertheless, the invention equally applies to the electrolysis of other alkali chlorides, such as lithium or potassium chloride. The mercury cathode-type electrolysis of sodium chloride solutions (hereinafter “brine”), based on a long-known technology, has experienced a continuous improvement in the cell structure (Ullmann's Encyclopaedia of Industrial Chemistry, VCH, Vol. A6, pag. 416) essentially directed to the decrease in the electric energy consumption and to the abatement of mercury emissions to the environment.
The problem of energy consumption reduction was accomplished with success by replacing the original graphite anodes with titanium anodes provided with a catalytic coating based on oxides of platinum group metals, particularly effective for chlorine evolution. The activated titanium anodes are also characterised by a long operative lifetime which allowed a substantial reduction in the amount of cell shut-downs which are quite frequent in the case of the corrodible graphite anodes. Since maintenance shut-downs are critical for mercury emissions, the benefit obtained from this standpoint is apparent.
A further reduction in mercury leaks was further allowed by modifications in the cell design, as disclosed in the co-pending Italian patent application MI2006A000309, and by the routine use of recrystallised salt, which permits minimising the quantity of mercury-polluted muds purged from the brine purification section, although at a higher cost.
As a consequence of these provisions it can be now be demonstrated that the release of mercury from a well-designed and correctly handled plant amounts to no more than 3 grammes per tonne of product chlorine versus a value of 10 grammes of about ten years ago (Ullmann's Encyclopaedia of Industrial Chemistry, VCH, Vol. A6, page 424).
In currently operating plants, the sodium amalgam is fed to the upper portion of an amalgam decomposer shaped as a generally vertical vessel containing a filling of graphite fragments activated with a catalyst, for instance molybdenum oxide. The amalgam flowing down the interstices between the graphite particles meets a countercurrent deionised water flow fed to the lower portion of the amalgam decomposer. The catalytic action of graphite allows the amalgam decomposition reaction to proceed at an acceptable rate, with formation of caustic soda, which together with chlorine is the product of commercial interest, according to the following scheme:Na(Hg)x+H2O→NaOH+½H2 
In reality, the mechanism accounting for the overall reaction is remarkably more complex as it involves the amalgam droplet—graphite particle couples statistically generated in time. Each of these couples actually operates as a short-circuited micro-battery, wherein the amalgam droplet and the graphite particle respectively act as anodic and cathodic areas according to the following partial reactions, whose combination gives the above indicated overall reaction:amalgam droplet(anodic area): Na(Hg)x→Na++xHg+e graphite particle(cathodic area): H2O+e→½H2+OH−
To obtain a high amalgam decomposition rate, it is necessary that the hydrogen evolution partial reaction, which is the bottleneck of the overall kinetics, be adequately promoted. For this reason, graphite, which has a moderate activity, is further added with more catalytic materials, such as molybdenum oxide, which is deposited by imbibition of an ammonium molybdate solution followed by a suitable thermal treatment. The so treated graphite is not wettable by the amalgam. Such feature has the advantage of impeding the formation of an adherent amalgam superficial film that would prevent the required contact with water which is the first step of the hydrogen evolution reaction.
On the other hand, this has also a negative effect, because the lack of wettability entails a high electrical contact resistance between graphite particle and amalgam droplet, in its turn strongly hindering the flow of electrons generated by sodium ionisation on the amalgam droplet and that must reach the graphite particle to adequately sustain hydrogen evolution. Hence it is shown that graphite cannot be an optimal catalyst, and this also applies for other homogeneous type formulations, such as the one disclosed in U.S. Pat. No. 4,161,433 wherein decomposition catalysts consisting of metal borides and carbides are proposed. From an ideal standpoint in fact any single particle of catalytic material should comprise one or more pairs of separate anodic and cathodic microscopic areas, the anodic ones being wettable by the amalgam and the cathodic ones being non wettable and in intimate contact with the water flow. It would be further necessary to minimise the electrical resistance between the two types of area to provide the lesser hindrance to the electrons flowing from the anodic areas, where they are released by sodium ionisation, to the cathodic areas where they are consumed in the hydrogen evolution reaction.
Besides the above mentioned inconveniences, the graphite particles undergo some grinding under the erosive action of the evolved hydrogen: as a first consequence, the filling undergoes a volume contraction in time that, once a value of about 20% is reached, leads to an intolerable productive capacity loss forcing the operators to discontinue the operation and to proceed with loading fresh catalyst.
From the above discussion it will be clear that the use of activated graphite represents a far from satisfying solution.